Chapter 7 Attaway MATLAB 4E Strings: Terminology A string in - PowerPoint PPT Presentation

String Manipulation Chapter 7 Attaway MATLAB 4E

Strings: Terminology — A string in MATLAB consists of any number of characters and is contained in single quotes — strings are vectors in which every element is a single character — A substring is a subset or part of a string — Characters include letters of the alphabet, digits, punctuation marks, white space, and control characters — Control characters are characters that cannot be printed, but accomplish a task (such as a backspace or tab) — White space characters include the space, tab, newline, and carriage return — Leading blanks are blank spaces at the beginning of a string, — Trailing blanks are blank spaces at the end of a string — Empty string is a string with length 0 , e.g. ''

String Variables — String variables can be created using — assignment statements — input function (with � s � as the second argument) — Since strings are vectors of characters, many built-in functions and operators that we � ve seen already work with strings as well as numbers – e.g., length to get the length of a string, or the transpose operator — You can also index into a string variable to get individual characters or to get subsets of strings, or in other words, substrings

String Concatenation — There are several ways to concatenate , or join, strings — To horizontally concatenate (creates one long string): — Using [ ], e.g. >> ['hello' 'there'] ans = hellothere — Using strcat , e.g. strcat( � hello � , � there � ) >> strcat('hello', 'there') ans = hellothere — There is a difference: if there are leading blanks, using [] will retain them whereas strcat will not

Vertical Concatenation — Vertically concatenating strings creates a column vector of strings, which is basically a character matrix (a matrix in which every element is a single character) — There are 2 ways to do this: — Using [ ] and separating with semicolons — Using char — Since all rows in a matrix must have the same number of characters, shorter strings must be padded with blank spaces so that all strings are the same length ; the built-in function char will do that automatically

Character Matrices — Both [ ] and char can be used to create a matrix in which every row has a string: >> cmat = ['Hello';'Hi '; 'Ciao ']; >> cmat = char('Hello', 'Hi', 'Ciao’); — Both of these will create a matrix cmat : H e l l o H i C i a o — Shorter strings are padded with blanks, e.g. cmat(2,:) is 'Hi '

String Functions — String functions that deal with blank spaces: — blanks creates a string of all blank spaces (can be useful in creating blank space in output by transposing; e.g. if 5 blank lines are desired, print blanks(5)’) — deblank removes trailing blanks — strtrim removes leading and trailing blanks — (Note: no functions remove blanks in the middle of strings) — String functions that convert case for letters: — upper converts a string to all upper case letters — lower converts a string to all lower case letters

The sprintf function — sprintf works just like fprintf , but instead of printing, it creates a string – so it can be used to customize the format of a string — So, sprintf can be used to create customized strings to pass to other functions (e.g., title , input ) >> maxran = randi([1, 50]); >> prompt = sprintf('Enter an integer from 1 to %d: ', maxran); >> mynum = input(prompt); Enter an integer from 1 to 46: 33 — Any time a string is required as an input, sprintf can create a customized string

Example Problem We will write a function � namedept � that will receive a name and department as separate strings and will create and return a code consisting of the first two letters of the name and the last two letters of the department. The code should be upper-case letters. For example, >> namedept('Robert', 'Mechanical') ans = ROAL

Example Solution function outcode = namedept(name, department) % Creates a code from a name and department % consisting of first two letters of the name % and the last two of the department outcode = upper(strcat(name(1:2), department(end-1:end))); end — Note: this is just one possible solution

String Comparisons — strcmp compares two strings and returns logical 1 if they are identical or 0 if not (or not the same length) — For strings, use this instead of the equality operator == — variations: — strncmp compares only the first n characters — strcmpi ignores case (upper or lower) — strncmpi compares n characters, ignoring case

Find and replace functions — strfind(string, substring) : finds all occurrences of the substring within the string; returns a vector of the indices of the beginning of the strings, or an empty vector if the substring is not found — strrep(string, oldsubstring, newsubstring) : finds all occurrences of the old substring within the string, and replaces with the new substring — the old and new substrings can be different lengths

The strtok function — The strtok function takes a string and breaks it into two pieces and returns both strings — It looks for a delimiter (by default a blank space) and returns a token which is the beginning of the string up to the delimiter, and also the rest of the string, including the delimiter — A second argument can be passed for the delimiter — So – no characters are lost; all characters from the original string are returned in the two output strings — Since the function returns two strings, the call to strtok should be in an assignment statement with two variables on the left to store the two strings

Examples of strtok >> mystring = 'Isle of Skye'; >> [first, rest] = strtok(mystring) first = Isle rest = of Skye >> length(rest) ans = 8 >> [f, r] = strtok(rest, 'y') f = of Sk r = ye

The eval function — The eval function evaluates a string as a function call or a statement — Usually used when the contents of the string are not known ahead of time; e.g., the user enters part of it and then a customized string is created — For example: >> x = 1:5; >> fn = input('Enter a function name: ', 's'); Enter a function name: cos >> eval(strcat(fn, '(x)')) ans = 0.5403 -0.4161 -0.9900 -0.6536 0.2837

eval example This is a very common application: a series of experiments has been run, resulting in files with the same name except for consecutive integers at the end of the name. We will write a for loop that will load files named � file1.dat � , � file2.dat � , … � file5.dat � (assuming that they exist) for i = 1:5 eval(sprintf('load file%d.dat',i)) end

“is” & String/Number Functions — � is � functions for strings: — isletter true if the input argument is a letter of the alphabet — isspace true if the input argument is a white space character — ischar true if the input argument is a string — isstrprop determines whether the characters in a string are in a category specified by second argument, e.g. ‘alphanumeric’ — Converting from strings to numbers and vice versa: — int2str converts from an integer to a string storing the integer — num2str converts a real number to a string containing the number — str2num ( and str2double) converts from a string containing number(s) to a number array — ( Note: different from converting to/from ASCII equivalents)

Common Pitfalls — Trying to use == to compare strings for equality, instead of the strcmp function (and its variations) — Confusing sprintf and fprintf . The syntax is the same, but sprintf creates a string whereas fprintf prints — Trying to create a vector of strings with varying lengths (the easiest way is to use char which will pad with extra blanks automatically) — Forgetting that when using strtok , the second argument returned (the “rest” of the string) contains the delimiter.

Programming Style Guidelines — Trim trailing blanks from strings that have been stored in matrices before using — Make sure the correct string comparison function is used; for example , strcmpi if ignoring case is desired